1. Field of the Invention
The present invention relates to a recording and playback apparatus for recording and playing back a variety of pieces of information to and from an optical recording medium, and an optical head for use in an optical apparatus such as the recording and playback apparatus, and a variable optical coupling efficiency device for use in the optical head.
2. Description of the Related Art
Optical recording media such as an optical disk have ruggedness (pits) representing signal information. Some recording media are used as playback purposes only. Some permit recording and playback operations using phase variation along a groove structure or using magneto-optical recording.
The optical head of a recording and playback apparatus such as an optical disk device which employs a write and read optical disk typically uses a semiconductor laser. The semiconductor laser light source typically gives a relatively high maximum optical output power to satisfy the following requirements.
(1) The semiconductor laser has difficulty in outputting a stable oscillation in a low output power level thereof, and laser noise becomes large in magnitude. To assure a required CNR (Carrier to Noise Ratio) during playback time, the output power of the laser must be set to be higher than a predetermined level. An laser output power of 2 to 5 mW is typically required.
(2) A writable optical recording medium performs a recording operation taking advantage of a rise in temperature of a recording layer due to an optical spot on the medium in which a light beam is focused. The recording power must be set to within a range that prevents the recording layer from being damaged by laser power during a playback operation while performing stable recording during a recording operation.
To satisfy these requirements, a difference in power must be permitted between during the playback operation and during the recording operation. A ratio of the maximum recording power/maximum playback power typically falls within a range of 5 to 20.
To perform recording at a speed higher than standard speed, a larger output power ratio is needed.
The maximum power rating of a light source of a write and read optical head is usually 20 to 50 mW to satisfy the above two requirements. A CD-R/RW that operates eight times as high as the standard speed has an output power of 100 mW.
A light source having a large optical output power is not only impracticable but also consumes much power.
If the maximum optical power rating is set to be low, no satisfactory playback characteristics are obtained because of a high noise level during the replay time.